Phase Interpolators (PIs) are used in high speed input-output circuits (I/Os). For example, PIs may be used in conjunction with a digital Low Pass Filter (LPF) to produce a digital Clock and Data Recovery (CDR) circuit, or by themselves to enable Design-for-Test (DFT) features such as the internal eye monitor system. Resolution, linearity, and jitter performance are some characteristics considered when designing a PI.
Traditional PIs are analog circuits using a Gilbert cell to mix two quadrature sinusoidal input clocks to produce a sinusoidal output clock with a desired phase offset. The sinusoidal output clock is classified as an analog clock. Many processors use digital clocks instead of analog clocks. Digital clocks refer to clock signals that have purely digital CMOS levels. To preserve linearity and step size, PIs using digital clocks convert the input digital CMOS level clocks to pseudo-sinusoidal (or triangular) shape for mixing. At the output of the PI, the sinusoidal shaped output is converted back to CMOS level. The input and output signal conversions use significant amount of power and area, and decrease the jitter performance and resolution of the PI.
To take advantage of purely digital CMOS clock, digital PIs (DPIs) mix different buffered versions of the input CMOS clocks to achieve a required delayed version of the CMOS clock output. However, due to variation of process corner, supply voltage and temperature (i.e., PVTs), many more phases than required are generated by the DPI. During a calibration procedure, only a sub-set of the phases are mapped for use in operation. This creates an overhead in area, power and calibration time. For example, each single phase needs to be measured and mapped at startup. Jitter performance is also compromised by such DPIs because the inverters used to produce the different slopes have slow transition times creating opportunity to inject power noise into the signal. Very high resolution is also very difficult to achieve.
Another issue related to the DPI is duty-cycle control. For example, to control duty cycle of the output signal from the DPI, the DPI slows down the slopes of the internal signal edges to compensate for duty cycle distortion. A reduced rising/falling slope leads to increased jitter performance and area.